Reference ladder auto-calibration circuit for an analog to digital converter

Abstract

An auto-calibrated reference resistor ladder network is disclosed which is suitable for use in a precision analog to digital converter. Resistors in the ladder are connected to an auto-calibration circuit which continually measures the potential developed across each of the connected resistors and adjusts the resistance of the resistor to match the measured potential to a reference potential. In the disclosed circuit, the reference potential is the potential developed across a defined one of the resistors.

I claim: 
     
       1. Apparatus which provides a plurality of reference voltages, comprising: a resistor ladder, having a first end and a second end coupled to respective first and second sources of reference potential, and having a plurality of serially connected resistors, each having first and second terminals and a resistance defined therebetween;   variable resistance means, coupled to one resistor of the serially connected resistors, for changing the resistance thereof responsive to a control signal;   comparison means, coupled to the one resistor, for generating a difference signal representing a difference between a predetermined reference potential and a potential measured between the first and second terminals of the one resistor; and   control means, responsive to the difference signal for generating the control signal for the variable resistance means, said control signal changing the resistance of the one resistor in a sense to reduce the difference signal in magnitude.   
     
     
       2. Apparatus according to claim 1, wherein the variable resistance means includes a transistor having a resistive channel defined between respective first and second electrodes, and a control electrode, responsive to the control signal for changing the resistivity of the resistive channel, wherein the first and second electrodes are coupled to the first and second terminals of the one resistor, respectively. 
     
     
       3. Apparatus according to claim 2, wherein the comparison means includes: means for periodically measuring the potential between the first and second terminals of the resistor; and   means for comparing the measured potential with the reference potential to provide a periodic signal as the difference signal; and   the control means includes means for integrating the difference signal to generate the control signal.   
     
     
       4. Apparatus according to claim 3, wherein the control means includes an integrating low-pass filter. 
     
     
       5. Apparatus according to claim 1, further comprising: a plurality of variable resistance means, each coupled to a respectively different one of the serially connected resistors, for changing the resistance thereof responsive to a respective one of a plurality of control signals; wherein: the comparison means includes means for generating a plurality of difference signals, each difference signal representing a difference between the reference potential and a respective potential measured between the first and second terminals of a respectively different one of said plurality of resistors; and   the control means includes means, responsive to said plurality of difference signals for generating the respective plurality of control signals, each of the control signals changing the resistance of the respective one of said plurality of resistances in a sense to reduce the respective difference signal in magnitude.   
     
     
       6. Apparatus according to claim 5, wherein the comparison means includes means for generating said plurality of difference signals sequentially, each of said difference signals being a periodic signal having an instantaneous magnitude which represents said difference at a predetermined instant. 
     
     
       7. Apparatus according to claim 6, further comprising auto-zero circuitry which applies a further reference signal as both the reference potential and the difference signal to the comparison means during periodic calibration intervals and which integrates any difference signal generated during the calibration interval to generate a bias potential for the comparison means to reduce, in magnitude, the difference signal sensed during subsequent ones of said calibration intervals. 
     
     
       8. An analog to digital converter (ADC) comprising: a voltage reference source which provides a plurality of voltage reference values, the voltage reference source including:   a resistor ladder, having a first end and a second end coupled to respective first and second sources of reference potential, and having a plurality of serially connected resistors, each having first and second terminals and a resistance defined therebetween, for providing a respective plurality of reference voltage values; and   autocalibration means, coupled to each of the plurality of resistors including:   means for selecting one of the plurality of resistors;   sampling means for measuring first and second ones of the plurality of reference voltage values provided at the first and second terminals of the selected resistor, respectively;   difference means for generating a value representing a difference between the first and second reference voltage values;   comparison means for comparing the generated difference value to a predetermined reference value to generate a control signal; and   control means, coupled to the selected resistor and responsive to the control signal, for changing the resistance defined between the first and second terminals in a sense to match the difference value to the predetermined reference value; and   means for comparing an input analog value to an analog signal derived from the plurality of reference voltage values to generate a digital value representing the input analog value.   
     
     
       9. Apparatus according to claim 8, wherein: the sampling means includes means for periodically measuring the first and second ones of the plurality of reference voltage values; the difference means includes means for generating a periodic signal representing differences between the first and second reference voltage values at respectively different instants; and   the comparison means includes means for comparing the generated periodic signal the predetermined reference value to provide a periodic signal as the control signal; and   the control means includes means for integrating the control signal to generate a further control signal which indicates an amount by which the resistance of the resistor is changed to match the difference value to the predetermined reference value.   
     
     
       10. Apparatus according to claim 8, further comprising: a plurality of variable resistance means, each coupled to a respectively different one of the serially connected resistors, for changing the resistance thereof responsive to a respective one of a plurality of adjustment signals; wherein: the comparison means includes means for generating a plurality of difference signals, each difference signal representing a difference between the predetermined reference potential and a respective potential measured between the first and second terminals of a respectively different one of said plurality of resistors; and   the control means includes means, responsive to said plurality of difference signals for generating the respective plurality of adjustment signals, each of the adjustment signals changing the resistance of the respective one of said plurality of resistances in a sense to reduce the respective difference signal in magnitude.